Summary of CIRCUIT PROVIDES REVERSE-BATTERY CONNECTION PROTECTION
This article describes a reverse-battery protection and automatic correction circuit that uses a low-on-resistance DPDT analog switch (IC1) as a full-wave rectifier to avoid diode voltage drops. Internal MOSFETs and ESD diodes ensure startup and fast (<20 ns) switching when battery polarity is reversed. Resistance per leg scales with battery voltage; typical total resistances: 5 Ω for four-cell packs and 10 Ω for two-cell packs. IC1 operates to 5.5 V at 30 mA and comes in a small 10-pin µMAX package, suitable for light- to medium-current portable devices.
Parts used in the Reverse-Battery Connection Protection:
- IC1: DPDT analog switch in 10-pin µMAX package (with internal MOSFETs and ESD-protection diodes)
- Battery (2-cell or 4-cell NiCd, NiMH, or alkaline)
- VCC terminal (system supply rail)
- Ground connection
- Control-pin connections for S1 and S2 (logic control to set normally closed/ open states)
A universal problem in battery-operated devices is the threat of damage when an end-user (never an engineer) inserts the battery backward. You can avoid damage by inserting a single diode or by using a diode-bridge configuration, but those fixes waste power and reduce the supply voltage by adding one or two diode drops between the battery and the supply rail. An alternative solution not only protects against battery-reversal damage but also automatically corrects the reversal (Figure 1). To eliminate the voltage drops associated with discrete diodes, a low-on-resistance, DPDT (double-pole, double-throw) switch serves as a full-wave rectifier. When you insert the battery with the correct polarity as shown, the upper switch, S1, is in its normally closed state, because its control pin is in its low state. The resulting connection from Pin 2 to Pin 10 provides a low-impedance path from the battery to the VCC terminal. Conversely, the lower switch, S2, closes its normally open terminal (not as shown) because its control pin is in its high state. The resulting path from Pin 7 to Pin 6 connects the battery’s negative terminal to ground.
The ESD-protection diodes in IC1 guarantee start-up and act as a full-wave rectifier (Figure 2). MOSFETs internal to the analog switch turn on when the battery voltage exceeds 1 V. Their less-than-20-nsec turn-on time enables the circuit to maintain normal operation by quickly swapping the leads of a reversed-polarity battery connection. The circuit resistance is proportional to the battery voltage. When the circuit operates from four NiCd, NiMH, or alkaline cells, the resistance in each leg of the rectifier is 2.5 Ω (5 Ω total). Operation with a two-cell battery (2.4 to 3 V) yields a total resistance of 10 Ω. IC1 is rated for operation to 5.5 V with 30-mA continuous current, making the circuit useful for cordless phones, portable audio equipment, handheld electronics, and other light- to medium-current applications. IC1‘s miniature 10-pin µMAX package takes less space than four through-hole signal diodes and is almost as small as two SOT-23 dual signal diodes.
Read more: CIRCUIT PROVIDES REVERSE-BATTERY CONNECTION PROTECTION
- How does the circuit protect against reversed battery insertion?
The DPDT analog switch IC1, using internal MOSFETs and ESD diodes, functions as a full-wave rectifier that reroutes battery leads to maintain correct polarity at VCC and ground. - Can the circuit automatically correct a reversed battery connection?
Yes; the MOSFETs turn on quickly and swap the leads to correct a reversed-polarity battery connection. - What enables startup when the battery is reversed?
ESD-protection diodes in IC1 guarantee startup and act as a full-wave rectifier until MOSFETs turn on. - How fast do the internal MOSFETs turn on?
The internal MOSFETs have a turn-on time of less than 20 ns. - What is the on-resistance or total resistance of the rectifier?
Resistance scales with battery voltage: about 2.5 Ω per leg (5 Ω total) for four-cell operation, and about 10 Ω total for two-cell operation. - What is the maximum operating voltage and current rating for IC1?
IC1 is rated for operation up to 5.5 V with 30 mA continuous current. - What applications are suitable for this circuit?
It is useful for cordless phones, portable audio equipment, handheld electronics, and other light- to medium-current applications. - How does this solution compare in size to discrete diodes?
The 10-pin µMAX IC1 occupies less space than four through-hole signal diodes and is nearly as small as two SOT-23 dual signal diodes.
